In general, it is well known in the papermaking industry that proper drainage of liquid from the paper stock on a forming fabric is an important step to ensure a quality product. This is done through the use of drainage blades or foils usually located at the wet end of the machine, e.g. a Fourdrinier paper machine. (Note the term drainage blade, as used herein, is meant to include blades or foils that cause drainage or stock activity or both.) A wide variety of different designs for these blades are available today. Typically, these blades provide for a bearing or support surface for the wire or forming fabric with a trailing portion for dewatering, which angles away from the wire. This creates a gap between the blade surface and the fabric, which causes a vacuum between the blade and the fabric. This not only drains water out of the fabric, but also can result in pulling the fabric down due to suction. However, when the vacuum collapses, the fabric returns to its original position, which can result in a pulse across the stock, which may be desirable for stock distribution. The activity (caused by the wire deflection) and the amount of water drained from the sheet are directly related to vacuum generated by the blade. Drainage and activity by such blades can be augmented by placing the blade or blades on a vacuum chamber. The direct relationship between drainage and activity is not desirable because while activity is always desirable, too much drainage early in the sheet formation process may have adverse effects on retention of fibers and filler. Rapid drainage may also cause sheet sealing, making subsequent water removal more difficult. Existing technology forces the paper maker to compromise desired activity in order to slow early drainage.
Drainage can be accomplished by way of a liquid to liquid transfer such as that taught in U.S. Pat. No. 3,823,062 to Ward, which is incorporated herein by reference. This reference teaches the removal of liquid through sudden pressure shocks to the stock. The reference states that controlled liquid to liquid drainage of water from the suspension is less violent than conventional drainage.
A similar type of drainage is taught in U.S. Pat. No. 5,242,547 to Corbellini. This patent teaches preventing the formation of a meniscus (air/water interface) on the surface of the forming fabric opposite the sheet to be drained. This reference achieves this by flooding the vacuum box structure containing the blade(s) and adjusting the draw off of the liquid by a control mechanism. This is referred to as “Submerged Drainage.” Improved dewatering is said to occur through the use of sub-atmospheric pressure in the suction box.
In addition to drainage, blades are constructed to purposely create activity in the suspension in order to provide for desirable distribution of the stock. Such a blade is taught, for example, in U.S. Pat. No. 4,789,433 to Fuchs. This reference teaches the use of a wave shaped blade (preferably having a rough dewatering surface) to create micro-turbulence in the fiber suspension.
Other types of blades wish to avoid turbulence, but yet affect drainage, such as that described, for example, in U.S. Pat. No. 4,687,549 to Kallmes. This reference teaches filling the gap between the blade and the web, and states that the absence of air prevents expansion and ‘cavitation’ of the water in the gap and substantially eliminates any pressure pulses. A number of such blades and other arrangements can be found in the following prior art: U.S. Pat. Nos. 5,951,823; 5,393,382; 5,089,090; 4,838,996; 5,011,577; 4,123,322; 3,874,998; 4,909,906; 3,598,694; 4,459,176; 4,544,449; 4,425,189; 5,437,769; 3,922,190; 5,389,207; 3,870,597; 5,387,320; 3,738,911; 5,169,500 and 5,830,322, which are incorporated herein by reference.
Traditionally, high and low speed paper machines produce different grades of paper with a wide range of basis weights. Sheet forming is a hydromechanical process and the motion of the fibers follow the motion of the fluid because the inertial force of an individual fiber is small compared to the viscous drag in the liquid. Formation and drainage elements affect three principle hydrodynamic processes, which are drainage, stock activity and oriented shear. Liquid is a substance that responds according to shear forces acting in or on it. Drainage is the flow through the wire or fabric, and it is characterized by a flow velocity that is usually time dependant. Stock activity, in an idealized sense, is the random fluctuation in flow velocity in the undrained fiber suspension, and generally appears due to a change in momentum in the flow due to deflection of the forming fabric in response to drainage forces or as being caused by blade configuration. The predominant effect of stock activity is to break down networks and to mobilize fibers in suspension. Oriented shear and stock activity are both shear-producing processes that differ only in their degree of orientation on a fairly large scale, i.e. a scale that is large compared to the size of individual fibers.
Oriented shear is shear flow having a distinct and recognizable pattern in the undrained fiber suspension. Cross Direction (“CD”) oriented shear improves both sheet formation and test. The primary mechanism for CD shear (on paper machines that do not shake) is the creation, collapse and subsequent recreation of well defined Machine Direction (“MD”) ridges in the stock of the fabric. The source of these ridges may be the headbox rectifier roll, the head box slice lip (see e.g., International Application PCT WO95/30048 published Nov. 9, 1995) or a formation shower. The ridges collapse and reform at constant intervals, depending upon machine speed and the mass above the forming fabric. This is referred to as CD shear inversion. The number of inversions and therefore the effect of CD shear is maximized if the fiber/water slurry maintains the maximum of its original kinetic energy and is subjected to drainage pulses located (in the MD) directly below the natural inversion points.
In any forming system, all these hydrodynamic processes may occur simultaneously. They are generally not uniformly distributed in either time or space, and they are not wholly independent of one another; they interact. In fact, each of these processes contributes in more than one way to the overall system. Thus, while the above-mentioned prior art may contribute to some aspect of the hydrodynamic processes aforesaid, they do not coordinate all processes in a relatively simple and effective way.
Stock activity in the early part of a Fourdrinier table as mentioned earlier is critical to the production of a good sheet of paper. Generally, stock activity can be defined as turbulence in the fiber-water slurry on the forming fabric. This turbulence takes place in all three dimensions. Stock activity plays a major part in developing good formation by impeding stratification of the sheet as it is formed, by breaking up fiber flocks, and by causing fiber orientation to be random.
Typically, stock activity quality is inversely proportional to water removal from the sheet; that is, activity is typically enhanced if the rate of dewatering is retarded or controlled. As water is removed, activity becomes more difficult because the sheet becomes set, the lack of water, which is the primary media in which the activity takes place, becomes scarcer. Good paper machine operation is thus a balance between activity, drainage and shear effect.
The capacity of each forming machine is determined by the forming elements that compose the table. After a forming board, the elements which follow have to drain the remaining water without destroying the mat already formed. The purpose of these elements is to enhance the work done by the previous forming elements.
As the basis weight is increased, the thickness of the mat is increased. With the actual forming/drainage elements it is not possible to maintain a controlled hydraulic pulse strong enough to produce the hydrodynamic processes necessary to make a well-formed sheet of paper.
An example of conventional means for reintroducing drainage water into the fiber stock in order to promote activity and drainage can be seen in FIGS. 1-4.
A table roll 100 in FIG. 1 causes a large positive pressure pulse to be applied to the sheet or fiber stock 96, which results from water 94 under the forming fabric 98 being forced into the incoming nip formed by the lead in roll 92 and forming fabric 98. The amount of water reintroduced is limited to the water adhered to the surface of the roll 92. The positive pulse has a good effect on stock activity; it causes flow perpendicular to the sheet surface. Likewise, on the exiting side of the roll 90, large negative pressures are generated, which greatly motivate drainage and the removal of fines. But reduction of consistency in the mat is not noticeable, so there is little improvement through increase in activity. Table rolls are generally limited to relatively slower machines because the desirable positive pulse transmitted to the heavy basis weight sheets at specific speeds becomes an undesirable positive pulse that disrupts the lighter basis weight sheets at faster speeds.
FIGS. 2 to 4 show low vacuum boxes 84 with different blade arrangements. A gravity foil is also used in low vacuum boxes. These low vacuum augmented units 84 provide the papermaker a tool that significantly affects the process by controlling the applied vacuum and the pulse characteristics. Examples of blade box configurations include:
Step blades 82 as show in FIGS. 2-3; and
Positive pulse step blade 78, as shown in FIG. 4, for example. Traditionally, the foil blade box, the offset plane blade box and the step blade box are mostly used in the forming process.
In use, a vacuum augmented foil blade box will generate vacuum as the gravity foil does, the water is removed continuously without control, and the predominant drainage process is filtration. Typically, there is no refluidization of the mat that is already formed.
In a vacuum augmented flat blade box, a slight positive pulse is generated over the blade/wire contact surface and the pressure exerted on the fiber mat is due only to the vacuum level maintained in the box.
In a vacuum augmented step blade box, as shown in FIG. 2 for example, a variety of pressure profiles are generated depending upon factors such as, step length, span between blades, machine speed, step depth, and vacuum applied. The step blade generates a peak vacuum relative to the square of the machine speed in the early part of the blade, this peak negative pressure causes the water to drain and at the same time the wire is deflected toward the step direction, part of the already drained water is forced to move back into the mat refluidizing the fibers and breaking up the flocks due to the resulting shear forces. If the applied vacuum is higher than necessary, the wire is forced to contact the step of the blade, as shown in FIG. 2. After some time of operation in such a condition, the foil accumulates dirt 76 in the step, losing the hydraulic pulse which is reduced to the minimum, as shown in FIG. 3, and prevents the reintroduction of water into the mat.
The vacuum augmented positive pulse step blade low vacuum box, as shown in FIG. 4, fluidizes the sheet by having each blade reintroduce part of the water removed by the preceding blade back into the mat. There is, however, no control on the amount of water reintroduced into the sheet.
Positive pulse blade, as water drains through the fabric, a converging nip produced by the lead angle of the blade and the fabric forces the water back into the sheet. This produces a shear force capable of breaking the fiber mat and penetrating through the stock slurry, re-fluidizing of the slurry is minimum, as it is shown in FIG. 5, for example.
A special type of double posi-blade incorporates a positive incoming nip to generate a positive and negative pressure pulse. This blade reintroduces water to the fiber mat with the lead in edge, the water reintroduced is limited to the amount adhere to the bottom of the forming fabric. This type of blade creates pressure pulses rather than consistency reduction. This type of blade simulates a table roll, as it is shown in FIG. 6, for example.
U.S. Pat. No. 5,830,322 to Cabrera et al., filed February 1996, titled “Velocity induced drainage method and unit” describes an alternate means of creating activity and drainage. The apparatus described therein decouples activity and drainage and thus presents a means of controlling and optimizing them. It uses a long blade with a controlled, probably non-flat or partially non-flat surface to induce initial activity in the sheet, and limits the flow after the blade through placement of a trail blade to control drainage. The '322 patent discloses that drainage is enhanced if the area between the long blade and forming fabric is flooded and surface tension is maintained between the water above and below the fabric. The invention disclosed therein is shown schematically in FIG. 7, for example.
However, with the '322 patent there is only one way to reintroduce a minimum amount of water to the fiber suspension. It occurs in the “counterflow zone,” and exists because the incompressible fluid follows the non-flat top of the long blade and is thus pumped through the forming fabric. The consistency that reaches the lead in edge of the Velocity Induce Unit does not change along the same blade. The stock consistency will be increased when the stock reaches the trial blade, because of drained water in the slot, if the Velocity Induce Unit is designed with multiple long blades and the consistency is constantly increased along the Velocity Induce Unit.
While some of the foregoing references have certain attendant advantages, further improvements and/or alternative forms, are always desirable.